A. Field of the Invention
The present invention relates generally to satellite systems, and more particularly, to satellite networks having devices that communicate with a low probability of detection.
B. Description of Related Art
Communication satellites represent one of the most significant applications of space technology. Communication satellites can allow radio, television, and telephone transmissions to be sent live anywhere in the world. Before satellites, transmissions were difficult or impossible at long distances. The signals, which travel in straight lines, could not bend around the Earth to reach a destination far away. Because satellites are in orbit, the signals can be sent instantaneously into space and then redirected to their destination.
Communication satellites are often in geostationary orbit. At the orbital altitude of 35,800 kilometers, a geostationary satellite orbits the Earth in the same amount of time it takes the Earth to revolve once. From Earth, therefore, the satellite appears to be stationary. Communication satellites can also be in highly elliptical orbits. This type of orbit is roughly egg-shaped, with the Earth near the top of the egg. In a highly elliptical orbit, the satellite's velocity changes depending on where it is in its orbital path. When the satellite is in the part of its orbit that is close to the Earth, it moves faster because the Earth's gravitational pull is stronger.
A system of satellites may be linked together to form a satellite network. In such networks, data may be relayed between multiple satellites and/or ground based devices before reaching its final destination. In some satellite networks, it may be desirable for satellites in the network to communicate with a low probability of detection. That is, it should be difficult for anyone other than the satellite's operators to know that the satellite is present, let alone communicating.
Conventionally, communications having a low probability of detection (LPD) may be implemented using techniques that include highly directional antennas, low power transmissions, sporadic “burst” transmissions, direct sequence and/or frequency-hopping spreading codes, or “noise-like” communications such as ultra-wideband or so-called “featureless” or “chaotic” waveforms. Many of these techniques, however, are difficult to apply to satellite systems. For example, highly directional antennas require good satellite stabilization and/or elaborate antenna structures. Low-power transmissions work poorly when attempting to communicate from a location in space to one on earth. Burst transmissions are not suitable for satellites that need to communicate in a more or less continuous manner. The featureless or chaotic waveforms require sophisticated radios, which can be problematic to implement on satellites.
Thus, it would be desirable to implement satellite networks that more effectively make it difficult to detect satellites or communications between the satellites.